Volcanoes are openings in a planet's surface that let out hot material from deep inside, shaping landscapes and impacting life.

Key idea: Most volcanoes are found at the edges of tectonic plates, where they either pull apart or push together, but some can form in the middle of plates at hotspots.

Most volcanoes on Earth are found where huge pieces of the Earth's outer shell, called tectonic plates, are either pulling apart or crashing into each other. Since much of the Earth's plate boundaries are under the ocean, many volcanoes are actually underwater.

For example, where plates pull apart, like at the Mid Atlantic Ridge, volcanoes form that usually erupt gently. Where plates crash together, like in the Pacific Ring of Fire, the volcanoes often have very powerful and explosive eruptions.

Sometimes, volcanoes can also form in the middle of a plate, far from any boundary. This happens when a super hot plume of rock rises from deep within the Earth, creating a 'hotspot.' The Hawaiian Islands are a great example of volcanoes formed by a hotspot.

“Most volcanoes on Earth are found where huge pieces of the Earth's outer shell, called tectonic plates, are either pulling apart or crashing into each other.”

Key idea: The word 'volcano' comes from Roman mythology, and the study of volcanoes is called volcanology.

The word 'volcano' comes from the Italian island of Vulcano. This island was named after Vulcan, the Roman god of fire.

The study of volcanoes and everything related to them is called volcanology. Scientists who study volcanoes are called volcanologists.

The processes and events involved in volcanic activity are known as volcanism.

Key idea: The movement of Earth's tectonic plates is the main reason why most volcanoes exist.

The Earth's outer layer, called the lithosphere, is broken into many large and small pieces, like a cracked eggshell. These are the tectonic plates. They are always moving, very slowly, because of the hot, flowing rock deep inside the Earth.

Most volcanic activity happens along the edges of these plates. When plates move apart, new crust is formed, and when they push together, one plate can slide under another, leading to volcanoes.

Some volcanoes erupt only once in their lifetime, while others erupt many times over long periods.

Key idea: Volcanoes at divergent plate boundaries form when plates pull apart, allowing magma to rise and create new crust, often underwater.

When two tectonic plates pull away from each other, hot rock from deep inside the Earth rises up. As this hot rock gets closer to the surface, the pressure drops, causing it to melt and form magma.

This magma then erupts, creating new ocean floor. Most of these volcanoes are deep underwater. Sometimes, these underwater volcanoes can grow so large that they break the surface of the ocean and form islands, like Iceland.

You might see 'black smokers' on the ocean floor, which are vents spewing hot, mineral rich water. These are signs of this kind of underwater volcanic activity.

Key idea: Volcanoes at convergent plate boundaries form when one plate slides under another, creating magma that often leads to explosive eruptions.

When two plates crash into each other, one plate often slides underneath the other. This process is called subduction. Usually, an oceanic plate slides under a continental plate.

As the oceanic plate sinks, it carries water deep into the Earth. This water lowers the melting point of the rock above it, creating magma. This magma is often thick and sticky, and it can create very explosive volcanoes when it reaches the surface.

These volcanoes often form long chains, like the ones around the Pacific Ocean, known as the Pacific Ring of Fire.

“When two plates crash into each other, one plate often slides underneath the other. This process is called subduction.”

Key idea: Hotspots are volcanic areas formed by rising plumes of hot rock in the Earth's mantle, creating chains of volcanoes as plates move over them.

Sometimes, volcanoes appear far from plate boundaries. These are called hotspots. Scientists believe hotspots are caused by very hot columns of rock, called mantle plumes, rising from deep within the Earth.

As a tectonic plate slowly moves over a stationary hotspot, new volcanoes form. The older volcanoes, no longer over the hotspot, become inactive. This creates a chain of volcanoes, like the Hawaiian Islands, where each island represents a volcano that formed over the hotspot and then moved away.

The Yellowstone Caldera in the United States is another example of a hotspot volcano.

Key idea: Volcanoes require a magma source, a path to the surface, and a vent, and they come in many different shapes and sizes.

A volcano needs three main things: a pool of molten rock (magma chamber), a path for the magma to travel up (conduit), and an opening to the surface (vent).

The material that erupts from the volcano, like lava and ash, builds up around the vent, forming the volcano's shape. This is often a cone or a mountain.

While many people imagine volcanoes as perfect cone shaped mountains with a crater at the top, there are actually many different types and shapes. Some have rugged peaks, others are flat plateaus, and some even have lakes inside their craters.

Key idea: Shield volcanoes are wide, gently sloped volcanoes formed by very fluid lava that flows easily.

Shield volcanoes get their name because they look like a warrior's shield lying on the ground: wide and gently sloping. They are formed by very fluid lava that flows easily and spreads out over a large area.

This type of lava usually does not explode violently. Instead, it flows out steadily. The volcanoes in Hawaii are famous shield volcanoes. Olympus Mons on Mars, the largest known volcano in our solar system, is also a shield volcano.

Key idea: Lava domes are steep, dome shaped volcanoes formed by thick, sticky lava that piles up around the vent.

Lava domes are built by very thick, sticky lava that does not flow far. Instead, it piles up around the vent, creating a steep, dome shaped mound. These can sometimes form inside the crater of a larger volcano.

Like some other types of volcanoes, lava domes can produce powerful, explosive eruptions, but the lava itself usually stays close to the vent.

Key idea: Cinder cones are small, cone shaped volcanoes built from loose volcanic rock fragments and ash.

Cinder cones are cone shaped hills made from small pieces of volcanic rock and ash that are thrown into the air during an eruption. These pieces, called cinders, pile up around the vent.

Cinder cones are usually smaller than other types of volcanoes and often erupt only once. Parícutin in Mexico is a famous example of a cinder cone that grew very quickly in a farmer's field.

Key idea: Stratovolcanoes are tall, cone shaped volcanoes made of alternating layers of lava and ash, known for their explosive eruptions.

Stratovolcanoes, also known as composite volcanoes, are tall, cone shaped mountains built from alternating layers of lava flows and ash. Think of Mount Fuji in Japan or Mount Vesuvius in Italy.

Their lava is usually thick and sticky, and it often contains a lot of trapped gas. This combination can lead to very violent and explosive eruptions, producing huge amounts of ash and dangerous fast moving flows of hot gas and debris called pyroclastic flows.

These volcanoes are steeper than shield volcanoes and can be very dangerous to nearby populations.

Key idea: Supervolcanoes are volcanoes capable of extremely rare, massive eruptions that can cause widespread devastation and global climate changes.

A supervolcano is a volcano that has had an eruption of truly enormous size, producing more than 1,000 cubic kilometers of volcanic material in a single event. These eruptions are incredibly rare but can cause devastation on a continental scale and even affect global temperatures for years.

When a supervolcano erupts, a huge underground magma chamber empties, causing the ground above it to collapse and form a giant basin called a caldera. Yellowstone National Park in the United States is home to a famous supervolcano caldera.

Because of their immense size and how long ago they erupted, supervolcanoes can be hard to spot without careful study.

Key idea: Volcanoes are classified as active, dormant, or extinct based on their eruption history and potential, though these categories can sometimes be unclear.

Volcanoes are often described as active, dormant, or extinct, but these terms can be a bit tricky and sometimes overlap.

An active volcano is one that is currently erupting or shows signs that it might erupt soon, like small earthquakes or gas emissions.

A dormant volcano is like a sleeping giant. It has not erupted for a long time, sometimes thousands of years, but it could still erupt again in the future. Mount Vesuvius, which destroyed Pompeii, was considered dormant before its famous eruption.

An extinct volcano is one that scientists believe will never erupt again because it no longer has a supply of magma. However, it can be very difficult to be absolutely sure a volcano is extinct, as some thought to be extinct have surprised everyone by erupting again!

Key idea: Volcanoes erupt gases, flowing lava, and solid fragments called tephra.

Volcanoes release three main types of material during an eruption:

1. Volcanic gases: Mostly steam, carbon dioxide, and sulfur compounds. These gases can be dangerous and affect the air quality.

2. Lava: This is what we call magma once it comes out of the volcano and flows over the surface. Lava can be thin and runny or thick and sticky, depending on its composition.

3. Tephra: These are solid pieces of volcanic material, like ash, dust, and larger rocks, that are blasted into the air. Ash can travel very far and cause problems for air travel and breathing.

Key idea: The thickness of lava, determined by its silica content, dictates whether an eruption will be explosive or flowing.

The type of lava a volcano erupts greatly affects its behavior. Lava with a lot of silica (a common mineral) is thick and sticky, like peanut butter. It traps gases easily, leading to explosive eruptions.

Lava with less silica is thinner and flows more easily, like syrup. This usually results in gentler, flowing eruptions.

For example, the runny lava from Hawaiian volcanoes is called 'basaltic' lava, while the sticky lava from explosive stratovolcanoes is often 'andesitic' or 'rhyolitic' lava.

Key idea: Volcanic eruptions are categorized by their explosivity, from gentle lava flows to super powerful blasts, using the Volcanic Explosivity Index.

Volcanic eruptions are classified by their style and how explosive they are. The Volcanic Explosivity Index (VEI) is a scale from 0 to 8 that measures an eruption's power.

Hawaiian eruptions (VEI 0) are gentle, with flowing lava. Strombolian eruptions are a bit more active, with frequent but small bursts.

Plinian eruptions are very powerful, creating huge columns of ash and gas that can reach high into the atmosphere. Supervolcano eruptions are the most intense, rated VEI 8.

Some eruptions, called phreatomagmatic, happen when rising magma meets groundwater, causing explosive steam blasts.

Key idea: Volcanoes are common throughout our solar system, with some planets and moons having ice volcanoes that erupt frozen materials.

Volcanoes are not just on Earth! Other planets and moons in our solar system also have them.

Venus has many volcanoes, and Mars has some enormous, extinct shield volcanoes, much bigger than any on Earth.

Jupiter's moon Io is the most volcanically active place in our solar system, constantly erupting with sulfur and other materials.

Some moons, like Europa and Triton, have 'ice volcanoes' or cryovolcanoes that erupt water, ammonia, or methane instead of molten rock.

Key idea: Our understanding of volcanoes evolved from mythical explanations to the scientific theory of plate tectonics.

For a long time, people did not understand why volcanoes erupted. Ancient cultures often believed they were caused by gods or mythical creatures.

Over time, thinkers started looking for natural explanations. Some thought eruptions were caused by strong winds or underground fires.

It was not until the 20th century, with the development of the plate tectonics theory, that scientists truly began to understand the forces behind volcanoes. This theory helped connect volcanoes to the movement of Earth's massive plates.